1. Field of the Invention
This invention relates to a light emitting element and a light emitting device using the same, and particularly relates to a light emitting element and a light emitting device that are provided with an improved external radiation efficiency in light radiated from the semiconductor layer.
2. Description of the Related Art
Conventionally, light emitting devices are known that an LED (light emitting diode) element is mounted on a substrate with a lead frame or a wiring pattern formed thereon. For such light emitting devices, it is important to increase the external radiation efficiency by reducing light confined in the LED element to offer high brightness and output.
The LED element used for the light emitting device includes a face-up type LED element. The face-up type LED element is structured such that p-type and n-type semiconductor layers including a light emitting layer are grown on a substrate such as a sapphire (Al2O3) substrate by vapor phase growth method, a passivation film for protecting the semiconductor layer or electrode is formed thereon so as to enhance the reliability, and the semiconductor layers side is used as the light radiation surface (light extraction surface).
Japanese patent application laid-open No. 6-291366 (prior art 1) discloses a face-up type LED element that has a passivation film of SnO2 to increase the external radiation efficiency. In prior art 1, the LED element is composed of a sapphire substrate and GaN-based compound semiconductor layers (with refractive index n=2.4) formed on the sapphire substrate, and its electrodes are disposed on the light radiation surface. The light radiation surface except for the electrodes is provided with SnO2 film (n=1.9) as a transparent electrode formed thereon, and the entire LED element is covered with a seal member of epoxy resin (n=1.5) to form a lamp type LED (FIG. 1 in prior art 1).
Further, the LED element used for the light emitting device includes a flip-chip type LED element. The flip-chip type LED element is structured such that semiconductor layers are formed on a transparent substrate such as a sapphire substrate and the transparent substrate side is used as the light radiation surface.
Japanese patent application laid-open No. 2002-219708 (prior art 2) discloses a flip-chip type LED element (LED chip) that an uneven face is provided on the light extraction surface side of substrate to reduce the light loss.
FIG. 1 is a cross sectional view showing an LED element disclosed in prior art 2. In the LED element 200, nitride semiconductor layers composed of a GaN buffer layer 202, an n-type semiconductor layer 203 and a p-type semiconductor layer 204 are formed on a sapphire substrate 201, a p-type electrode 205 is formed on the p-type semiconductor layer 204, and an n-type electrode 206 is formed on the n-type semiconductor layer 203. The LED element 200 is flip-chip bonded through bumps 230a, 230b onto a mount board 210. On the opposite face of sapphire substrate 201 to the surface thereof with the nitride semiconductor layers formed, uneven surfaces 201a, 201b of 1 μm or so are formed by polishing the opposite face while adjusting the grain size of an abrasive (paragraphs [0022]-[0024] and FIG. 2 in prior art 2).
However, the conventional LED elements have the next problems.
(1) If the optical distance (the product of optical path length and medium refractive index) of film thickness is ¼ or [(2m+1)/4; m is an integer] times of emission wavelength, of light coming from the GaN-based compound semiconductor layer to the SnO2 film, perpendicular incident light has an phase difference to light reflected at the interface of epoxy resin and SnO2 film that causes to reduce the interface reflection light and to increase the interface transmission light in light coming from the GaN-based compound semiconductor layer to the SnO2 film. Therefore, the external light extraction efficiency can be enhanced. In like manner, light with such an incident angle that the optical distance in the SnO2 film (the optical distance of light entered into the SnO2 film from the GaN-based compound semiconductor layer, reflected at the interface of epoxy resin and SnO2 film, returned to the SnO2 film and the GaN-based compound semiconductor layer) becomes ¼ or [(2m+1)/4; m is an integer] times of emission wavelength has such an phase difference that causes to reduce the interface reflection light and to increase the interface transmission light. However, if the thin film does not have a large value of m, such light entered to this specific direction from the interface only occupies a small part of all light emitted from the light emitting layer.
On the other hand, in the case of light subjected to total reflection when being entered at an angle greater than its critical angle to the SnO2 interface from the GaN-based compound semiconductor layer, the abovementioned effect of SnO2 film is not obtained because return light as interference light to this light is not generated at the interface of the SnO2 film and the epoxy resin. Provided that light emitted from the light emitting layer is regarded as perfect diffusion light and is externally emitted only from the upper surface, light from the GaN-based semiconductor layer to be subjected to total reflection at the SnO2 film interface occupies about 65% of all light.
(2) In practical use, the LED element disclosed in prior art 2 is generally sealed with epoxy resin with a refractive index of 1.5. In this case, the light extraction efficiency (external radiation efficiency) can be little improved even when the surface of sapphire substrate with a refractive index of 1.7 is roughened. Thus, most of light will be confined in the semiconductor layers and therefore the light emitting device will not offer a high brightness.